This invention relates generally to the field of oil and gas exploration. More particularly, this invention relates to the use of aqueous and oil inclusions in rocks to reconstruct paleo-pressure history to evaluate hydrocarbon potential of sedimentary basins.
A critical component of petroleum exploration involves characterizing the risk associated with the timing of hydrocarbon migration and trap fill, reservoir pressure history, and timing of seal failure. This requires reliable estimates of paleo-pressure. Paleo-pressure is the ancient pressure of the sedimentary basin (i.e., the pressure during formation of the sedimentary basin). Paleo-pressure estimation has proven to be difficult to quantify and there is a need for an accurate and low-cost method of measuring paleopressure directly from geological samples.
The only known technique for directly measuring geological paleopressure utilizes fluid inclusions in rocks. Fluid inclusions are microscopic samples of paleo-fluids that are trapped and sealed within cavities in minerals. These inclusions preserve the pressure at which they were trapped. Several methods have been used to obtain trapping pressure from fluid inclusions in petroleum systems. For example, pressure from individual methane-carbon dioxide (CH4xe2x80x94CO2) gas inclusions can be obtained by using Raman spectral parameters (Seitz, J. C., Pasteris, J. D., and Chou I-M., 1996. Raman spectroscopic characterization of gas mixtures. II. Quantitative composition and pressure determination of the CO2xe2x80x94CH4 system. American Journal of Sciences, 296, 577-600). The underlying principle of Raman spectral analysis is when monochromatic (i.e. substantially single photon energy) light, such as emitted from a laser traverses a medium (e.g. gas, liquid, or solid) the majority of the scattered light will remain at the incident photon energy. However, a small proportion of the scattered light will be at changed frequencies, above and below the incident photon energy, and this is referred to as anti-Stokes and Stokes Raman scattering, respectively. The energy increment and decrement for the anti-Stokes and Stokes scattering, respectively correspond to the vibrations of the molecules of the medium that produce the scattered photons.
By measuring the energy decrement of the scattered light relative to the incident light, Raman spectroscopy is a tool to probe molecular vibrations. As Raman spectroscopy may be carried out within the ultra-violet (UV) and visible regions of the spectrum, the incident laser beam can be focused by normal light optics, i.e. microscope objectives, to give spatial resolution in the region of 1 micrometer. It therefore provides a non-destructive means of analyzing the molecular species of very small objects, including fluid inclusions in minerals. This technique has proven to be particularly successful for the analysis of species such as carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethane (C2H6), nitrogen (N2), water (H2O), hydrogen sulfide (H2S), hydrogen sulfite (H2SO3), oxygen (O2), and sulfates (SO4xe2x88x922).
Laser photon energies in the range of 50000 cmxe2x88x921 to 9000 cmxe2x88x921 may be used for excitation. The Raman scattered radiation is detected over a range of 150 cmxe2x88x921 to 4600 cmxe2x88x921 below the laser excitation energy. Preferably, the inclusion to be analyzed is illuminated through a microscope and a portion of the backscattered laser excitation and Raman emission are collected and collimated by the objective. Other angles between incident and scattered laser light may be used. Filters separate the laser and Raman components and the latter is recorded by a spectrometer.
Inclusion trapping pressure from Seitz et al. method can be obtained from the peak positions of the CH4 and CO2 Raman bands obtained from gas inclusions. However, this application is limited to pure CH4, CO2 and/or CH4xe2x80x94CO2-bearing inclusions, which are rare for petroleum systems. Raman spectroscopy was also used to obtain minimum trapping pressure from synthetic CH4-bearing aqueous fluid inclusions (Dubessy, J., Pironon, J., Lamb, W., McShane, C., Popp, C., Thiery, R., 1998. PACROFI VII, Jun. 1-4, 1998, 27, Leng, J., Sharma, A., Bodnar, R. J., Pottorf, R. J., Vityk, M. O., 1998. Quantitative Analysis of Synthetic Fluid Inclusions in the H2Oxe2x80x94CH4 System Using Raman Spectroscopy. PACROFI VII, Jun. 1-4, 1998, 41).
The method of Dubessy et al. uses microthermometry to determine the salinity of the aqueous phase, and Raman spectra of methane and water and the thermodynamic properties of the water-methane (H2Oxe2x80x94CH4), no salt, system to model the inclusion methane content. Microthermometry is the observation of the temperatures of phase changes within fluid inclusions as they are cooled or heated on a special microscope stage. This method can provide two basic kinds of information: inclusion composition and temperature of entrapment. The temperature of ice melting tells us something about chemistry of inclusion fluid and the temperature of the inclusion bubble (gas phase) disappearance (homogenization temperature) is a minimum or true value for the temperature at which the inclusion was trapped. The inclusion composition and homogenization temperatures are used to obtain the pressure at the bubble point. Bubble point pressure is the pressure at inclusion saturation or saturation pressure. However, the Dubessy et al. method has a number of limitations.
One of these limitations is that this method can not be used to obtain the true trapping pressure. Reliable pressure measurements from fluid inclusions are entirely dependent on the specific type of fluid inclusions measured. Some fluid inclusions can be used to determine only a minimum pressure of entrapment, some may be used to determine the true pressure of entrapment. Fluid inclusions that can be used to determine the true pressure of entrapment need to be recognized in the rock based on a unique set of petrographic and microthermometric criteria. The Dubessy et al. method provides no such criteria.
Additional limitations of the Dubessy et al. method include 1) lack of calibration with application to natural petroleum systems of interest which contain salt and generally homogenize at temperatures below 300xc2x0 C., and 2) the necessity for additional calibration for the effect of salinity. Application of the method to determine the potential for hydrocarbons is not discussed.
The work of Leng et al. reports the Raman band area ratio for synthetic CH4- water (no salt) inclusions and discusses neither the determination of the formation pressure from the Raman data nor the use of the pressure determination for hydrocarbon exploration applications.
The bubble point pressure or saturation pressure can be obtained from individual oil inclusions by using confocal scanning laser microscopy coupled with microthermometry (Pironon, J., Canals, M., Dubessy, J., Walgenwitz, F., Laplace-Builhe, C., 1998. Volumetric Reconstruction of Individual Fluid Inclusions By Confocal Scanning Laser Microscopy. Eur. J. Mineral. 10, 1143-1150, Aplin, A. C., Macleod, G., Larter, S. R., Sorensen, H., Booth, T, 1999. Combined Use of Confocal Laser Scanning Microscopy and PVT Simulation For Estimating the Composition and Physical Properties of Petroleum in Fluid Inclusions. Mar. Petrol. Geol. 16, 97-100, Aplin, A. C., Larter, S. R., Bigge, M. A., Macleod, G., Swabrick, R. E., Grunberger, D., 2000, Confocal microscopy of fluid inclusions reveals fluid pressure histories of sediments and an unexpected origin of gas condensate. Geology, no. 11, 1047-1050). This method involves generation of three-dimensional images of an individual oil inclusion by using confocal scanning laser microscopy and calculation of the volumetric ratio of oil to gas within the inclusion. Using commercial software (Aplin et al., 1999), these data along with inclusion homogenization temperature (Th) are used to reconstruct the bubble point for the inclusion oil and to obtain the pressure at the homogenization temperature, which is the minimum trapping pressure. The true trapping pressure can be obtained if coexisting aqueous inclusions are present in the sample by using the xe2x80x9ccrossing isochore techniquexe2x80x9d (Roedder, E., and Bodnar, R. J., 1980. Geologic Pressure Determination From Fluid Inclusion Studies. Ann. Rev. Earth. Planet. Sci. 8, 263-301).
One limitation of the confocal method is that the accuracy of the bubble point pressure calculation is heavily dependent on the initial oil composition entered by the user. Pressure-temperature xe2x80x9cP-Txe2x80x9d reports on nearby reservoired oils are presently the best data available to infer the P-T properties of oil inclusions. However, it is understood that inclusions trapped in the geological past may not be similar to present-day reservoired oil. Also, for many frontier locations these P-T reports are not available. An additional limitation of this method is that it can be used only for oil inclusions. This limits the application of the method mostly to the systems that contain oil. This method cannot be used for gas systems and for non-hydrocarbon systems. A method is needed for more robust and reliable pressure determination from fluid inclusions in sedimentary basins.
After an oil leg is penetrated during a drilling operation, there is the need to evaluate nearby gas resources. The conventional method of identifying gas cap from an oil leg involves down-hole sampling of reservoir oil and measuring its P-T properties. Reservoir fluid sampling is very expensive and new inexpensive methods, such as fluid inclusion based techniques, are needed for evaluating nearby gas accumulations.
Accordingly, a need exists for a reliable method for using fluid inclusions in rocks to reconstruct paleo-pressure history to evaluate hydrocarbon potential of sedimentary basins which overcomes the problems inherent in prior techniques. The present invention satisfies this need.
A method, based upon spectroscopy (e.g. Raman spectroscopy) and microthermometry of aqueous inclusions which are very common in sedimentary rocks, is described which results in more robust and reliable estimation of the paleo-pressure and paleo-salinity of fluids in sedimentary basins.
One embodiment is a method of determining pressure at the time of formation of fluid inclusions contained in sedimentary rocks. This method comprises; (a) identifying a fluid inclusion in a rock sample for analysis; (b) measuring the homogenization temperature of said fluid inclusion; (c) measuring the optical spectrum of methane and water in said fluid inclusion; (d) determining the relationship of homogenization temperature and optical spectrum to; formation pressure for said fluid inclusion; and (e) estimating formation pressure of said fluid inclusion from said relationship and said measurements of homogenization temperature and optical spectrum.
In another embodiment, the invention comprises (a) preparing samples to obtain both high quality images of the fluid phases in individual inclusions and optical spectra (i.e. ultraviolet, visible, infrared) of the fluid components with sufficient discrimination from fluorescence, Raman bands from non-inclusion material, and other interfering optical radiation; (b) determining the homogenization temperature at which the aqueous inclusion fluid components homogenize into a single fluid phase; (c) deter the relative methane composition of the inclusions from the spectra; (d) obtaining the salinity of the aqueous inclusions from the spectra; (e) using the methane concentration, salinity from aqueous inclusions and homogenization temperature from aqueous, oil and/or gas inclusions to estimate the physical conditions, such as pressure, and the geochemical environments in which the fluids were trapped; and (f) relating this information to the evaluation of sedimentary basins for their hydrocarbon potential and quality. Preferably, the spectra used to determine methane composition of the inclusions are obtained at homogenization temperature.
The invention is described with reference to aqueous fluid inclusions from core, cuttings, and outcrop samples, henceforth called rock fragments, obtained from petroleum environments. The inclusions of interest to the instant invention are formed in optically transparent minerals such as quartz, carbonate, feldspar, halite, fluorite or salt. For these inclusions, methane is the predominant, but not necessarily the only, hydrocarbon fluid and is generally a minor (approximately less than 10 percent weight, xe2x80x9cwt. %xe2x80x9d) constituent in water. The water may contain 0 to 25 wt. % or more of NaCl or other salts. The inclusion fluids typically homogenize to a single phase at temperatures below 300xc2x0 C.
The spectroscopy used for the method may be Raman spectroscopy, or any other suitable optical spectroscopy, in the ultraviolet, visible, or infrared portions of the electromagnetic spectrum. The only requirement is that vibrational bands of the methane and water in the individual inclusion can be determined and related to pressure.
Salinity may be measured by either of two novel methods involving spectroscopy. One method is the pattern recognition method, the other method is the chemometric method. Less preferably, salinity may be determined by conventional freezing point depression methods, which are known in the art.
The pressure obtained from fluid inclusions can be used to evaluate hydrocarbon potential of sedimentary basins. In particular, pressure from fluid inclusions can be applied to evaluate paleo and present-day pressure, timing of hydrocarbon migration, timing of seal failure and likelihood of a gas cap in a reservoir.